The present invention relates to the structure of a collector included in a radiation cooling type high output travelling-wave tube mounted on a satellite.
Travelling-wave microwave tubes for satellite applications are extensively used for satellite broadcasting and microwave communication using satellites. This kind of tube includes an electron gun, a wave delay circuit, and a collector. While the electron gun emits an electron beam, the wave delay circuit substantially equalizes the phase velocity of an electromagnetic wave to the electron velocity of the electron beam. The collector transforms the kinetic energy of the electron beam to heat, and radiates the heat to the outside. To insure the long-term high-output operation of the tube in the space, it is necessary that heat output from the collector be prevented from elevating the temperature of the body of a satellite on which the tube is mounted.
In light of the above, use has customarily been made of a radiation cooling type travelling-wave tube having a collector core protruded to the outside of the structural body of a satellite. In this condition, heat output from the collector is directly radiated into the space with the result that a thermal load on the collector is reduced. For example, the collector or heat radiating means of this type of tube has a collector core formed of copper, collector electrodes disposed in the core, and a ceramic coating film covering the outer periphery of the core. The heat radiation effect available with the tube is expressed in terms of the emissivity .epsilon. of the ceramic coating film. The emissivity .epsilon. is an extremely important factor because the tube mounted on a satellite is operated in space.
Table 1 shown below lists some specific emissivities of the ceramic coating film.
TABLE 1 ______________________________________ SAMPLE FILM THICKNESS .mu.m EMISSIVITY .epsilon. ______________________________________ MgO--AL.sub.2 O.sub.3 500 0.85 (Magnesia--Alumina) TiO.sub.2 --AL.sub.2 O.sub.3 500 0.86 (Titania--Alumina) Cr.sub.2 O.sub.3 500 0.86 (Chromium Oxide) ______________________________________
For a coating method using flame-spraying, a 500 .mu.m thick ceramic coating film is generally used. A series of studies by the present inventor showed that the thickness of 500 .mu.m is optimal. Specifically, thicknesses greater than 500 .mu.m caused the film to easily comes off while thicknesses smaller than 500 .mu.m reduced the emissivity in proportion thereto.
As shown in Table 1, the emissivity .epsilon. was found to be 0.85 with a magnesia-alumina sample, 0.86 with a titania-alumina ceramic sample, or 0.86 with a chromium oxide sample. That is, the maximum emissivity .epsilon. available with ceramic coating films is 0.86.
However, in parallel with the increasing output of the tube for use in a satellite, the required emissivity is increasing. Extended studies by the present inventor showed that an emissivity .epsilon. of greater than or equal to 0.90 is essential in order to insure the long-term operation of the tube in space. In this respect, the emissivity .epsilon. achievable with the above conventional ceramic coating films cannot implement the sufficient heat radiation currently required of the collector of the tube. Moreover, the film usually 500 .mu.m thick is apt to crack or come off when subjected to mechanical vibration.
To radiate the heat output from the collector, the collector may be painted or provided with an organic thin film thereon. This kind of scheme, however, brings about a critical problem that the emissivity falls due to its limited resistivity to ultraviolet rays and cosmic dust.
In this manner, considering the application of the collector or radiator to satellites, the collector must be reduced in weight, be reliable under the severe environmental conditions including mechanical vibration and temperature, and in addition be stable with respect to heat radiation and resistivity to ultraviolet rays.
On the other hand, Japanese Patent Laid-Open Publication No. 63-45895, for example, teaches a method capable of providing an aluminum circuit board with a high heat radiating ability and insulating ability by reducing the thickness of an adhesive resin layer. A technology of the kind forming an insulating oxide film (sulfate film) on an aluminum surface by sulfuric anodization and forming an adhesive resin film via the sulfate film is conventional. The problem with this kind of technology is that the adhesion between the resin layer and the circuit board, particularly during heating, is too weak to prevent copper foil or a similar member from coming off during, e.g., soldering of circuit parts. The method taught in the above document is a solution to this problem. Specifically, to insure adhesion between the circuit board and the resin layer and therefore copper foil or the like, the method is characterized in that the surface of the circuit board is roughened to the maximum surface roughness Rmax of 8+3 .mu.m, and then a 3 .mu.m to 20 .mu.m thick oxide film is formed on the roughened surface by anodization.
The present inventor has applied the above prior art method to the fins of a collector. Specifically, a 3 mm thick aluminum film formed of JIS (Japanese Industrial Standard) 1100 alloy had its surface roughened to the maximum surface roughness Rmax of 5 .mu.m to 11 .mu.m, and then the roughened surface was anodized to form a 20 .mu.m oxide film. The experiment showed that the maximum emissivity .epsilon. available with such fins is only 0.81 which is even lower than the emissivity of the ceramic coating film. Therefore, this kind of scheme alone cannot provide a collector with the required emissivity alone.